That could happen if you are using a LIN pot instead of a LOG pot, or if you have the LOG pot wired backwards.

I did use a 500k LIN pot indeed, because I did not have a LOG type, so it's a long travel from 50k to 500k and with a LIN pot nothing much interesting happens in between.The most interesting sounds are really in the 47k, 100k and 500k positions, and I will eventually go for the 3 position switch to keep it a true "Tri-Vibe".

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I guess the size of this circuit is as much as you can pack in such space without going to surface mount devices.

Yes, for me it probably is.I made a PCB with the same size for Mark Hammer's "The Woody" and with 2 Quad opamps and quite a lot of caps and resistors that also was a tight fit,but I see it as a challenge to make rather big circuits fit in a 1590B enclosure.

I'm in the process of building the Vibro-Matic III from the Stompbox Cookbook, and the circuit looks fairly similar to this Tri-Vibe. I listened to the soundclips and they sound different, but not radically so. Do you think it would be worth it to build a Tri-Vibe in addition to the Vibro-Matic III? I have around 7 overdrives on my board, so 2 different vibratos wouldn't be pushing it, right?

Well...I had done my own layout but I wasn't happy with it and I boxed myself in in a couple places.I redrew it with Andre's layout as the main framework, then moved some things around.It's almost finished...Stay tuned.

having trouble here with your posted layout. The signal passes the circuit unmodulated though checking the voltage tells me that the lfo is clearly working. I have to double check my build and your layout w